Method for extracting tungsten from scheelite

ABSTRACT

A method for extracting tungsten from scheelite by: 1) adding a mixed acid including H 2 SO 4  and H 3 PO 4  to a decomposition reactor; 2) heating the mixed acid to a temperature of 70-100° C.; adding scheelite while controlling the liquid-solid ratio at about 3:1-8:1 L/kg; allowing the components in the decomposition reactor to react for 1-6 h, and filtering to obtain a filtrate; 3) supplementing the filtrate with sulfuric acid consumed in the reaction; 4) crystallizing the filtrate to obtain phosphotungstic acid crystals and mother liquor; 5) dissolving the phosphotungstic acid crystals in water to obtain phosphotungstic acid solution; 6) transforming the phosphotungstic acid solution into an ammonium tungstate solution for the purpose of preparing ammonium paratungstate; and 7) supplementing the mother liquor with phosphoric acid and water to an initial level and reusing the mother liquor for ore leaching.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International Patent Application No. PCT/CN2011/001895 with an international filing date of Nov. 11, 2011, designating the United States, now pending, and further claims priority benefits to Chinese Patent Application No. 201010605094.7 filed Dec. 24, 2010, to Chinese Patent Application No. 201010605095.1 filed Dec. 24, 2010, to Chinese Patent Application No. 201010605103.2 filed Dec. 24, 2010, to Chinese Patent Application No. 201010605107.0 filed Dec. 24, 2010, and to Chinese Patent Application No. 201010605110.2 filed Dec. 24, 2010. The contents of all of the aforementioned applications, including any intervening amendments thereto, are incorporated herein by reference. Inquiries from the public to applicants or assignees concerning this document or the related applications should be directed to: Matthias Scholl P.C., Attn.: Dr. Matthias Scholl Esq., 14781 Memorial Drive, Suite 1319, Houston, Tex. 77079.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a hydrometallurgical field, and more particularly to a method for extracting tungsten from scheelite.

2. Description of the Related Art

A typical method for processing scheelite is autoclaving using suda as a leaching agent. The method can stably decompose scheelite, and a final WO₃ in a residue can be controlled 1% below; however, the practical dosage of the reagent is too large which is about three times of the theoretical dosage. Sometimes in the lab experiment, the practical dosage is even 5-6 times of the theoretical dosage. Furthermore, the operation temperature is very high, about 225° C., and the pressure of the device is 20 atm.

Another method for processing scheelite is autoclaving using NaOH as a leaching agent, and more than 80% of APT production of China is produced by this method. The method is carried out with a large amount of NaOH having a high concentration being added, at a high temperature and pressure, so that the scheelite can be effectively decomposed, and WO₃ in the residues can be lowered to 1-3%. However, the technique has a large energy consumption, high production cost, and large amount of waste water in the subsequent process.

Acid decomposition is a method that mainly uses hydrochloric acid to process scheelite concentrate. A thermodynamic study on such a method has proved that it has a very high reaction trend. However, during the hydrochloric acid decomposition, tungstic acid is in a form of yellow gel wrapping an inner undecomposed scheelite, which results in incomplete decomposition. Besides, the hydrochloric acid has serious problems of acid corrosion and volatilization, which causes a poor working condition. Finally, after treating the remnant mother liquor of hydrochloric acid with a lime, a large amount of CaCl₂ solution produced is discharged. This method has been discarded.

Because tungsten together with phosphorus, arsenic, silicon, and other impurities can form a soluble heteropoly acid (for example, [PW₁₂O₄₀]³⁻) having 1:6-1:12 of a ratio of impurities to tungsten; during the hydrochloric acid decomposition, even a small amount of phosphorus can cause loss of a large amount of tungsten into leaching solution, so that the hydrochloric acid decomposition is mainly used to process a scheelite concentrate having a high purity (which is required to have a very low content of phosphorus, arsenic and other impurities). However, it implies us that adding a small amount of phosphoric acid intentionally during the leaching process to cause tungsten into the solution in the form of soluble phosphotungstic heteropoly acid, the problem of tungstic acid wrapping during the hydrochloric acid decomposition can be overcome. But study has shown that the yellow tungstic acid still occurs when the dosage of phosphorus is too small, so that a large excess coefficient is needed, and the more phosphorus is used, the higher the leaching speed is. However, corrosion and volatilization problems of the hydrochloric acid still exist, and thus, the study has not been applied in industry.

To solve corrosion and volatilization problems of hydrochloric acid, and to realize the leaching of the tungsten in the form of soluble phosphotungstic heteropoly acid, sulfuric acid is considered to substitute hydrochloric acid. However, when a large amount of sulfuric acid exists, the supersaturated gypsum is quickly nucleated to form a large amount of finely crystals, which results in wrapping, and the decomposition effect is not good. During the sulfuric acid decomposition, phosphoric acid, calcium phosphate, or phosphorite is added to provide phosphorus as a complexing agent for tungsten, but a certain amount of NaCl is needed to improve the decomposition. Then, HCl in the strong sulfuric acid solution has a high degree of activity, which is equal to a high concentration of a hydrochloride acid, thus, problems like the corrosion of C⁻ come out again.

The principle of the action of NaCl, is that the chloride ions can largely increase the induction period of the calcium sulfate crystals, thereby preventing the calcium sulfate crystals from nucleating. To a certain degree, it is helpful to form large crystals, and prevent the products from wrapping the minerals and obstructing the decomposition. However, in actual fact, adding NaCl cannot achieve an ideal decomposition result, but bring up problems like device corrosion and HCl volatilization; besides, it cannot solve the problem of tungstic acid wrapping.

SUMMARY OF THE INVENTION

Our study shows that, using a phosphoric acid solution having a high concentration (a concentration of P₂O₅ is 15-35%) to decompose a scheelite can largely improve the formation rate of a soluble of phosphotungstic acid, avoiding wrapping of the tungstic acid precipitation, as shown in equation (1).

12CaWO₄+25H₃PO₄=12Ca(H₂PO₄)₂.H₂O+H₃[PW₁₂O₄₀]+11H₂O   (1)

Furthermore, phosphoric acid has a low corrosive effect, and no volatilization problem like hydrochloric acid.

However, phosphoric acid has a high production cost, and a large amount of phosphorus exist in a form of Ca(H₂PO₄)₂.H₂O, which leads to difficulties in carrying out further reactions. Thus, during the decompostion process, a certain amount of H₂SO₄ can be added to react with the consequent Ca(H₂PO₄)₂.H₂O, so that calcium is finally precipitated in a form of calcium sulfate, and phosphoric acid is formed again, thereby lowing the consumption of phosphoric acid. The equation (2) is as follows:

Ca(H₂PO₄)₂.H₂O+H₂SO₄→CaSO₄.nH₂O+H₃PO₄   (2)

A total equation is:

12CaWO₄+H₃PO₄+12H₂SO₄+12nH₂O =12CaSO₄.nH₂O+H₃[PW₁₂O₄₀]+12H₂O   (3)

Furthermore, the phosphoric acid solution having a high concentration can effectively lower the degree of supersaturation of calcium sulfate during the decomposition of tungsten ore. This is because phosphoric acid can complex calcium ions, thereby the solubility of the calcium sulfate increases as the concentration of the phosphoric acid increases, and reach a highest when the P₂O₅ is about 20 wt. %. The solubility of the calcium sulfate at the temperature of 80° C. is 5-7 times as those having no phosphoric acid. Even when P₂O₅ reaches to 40%, the solubility of the calcium sulfate is 3-5 times as those having no phosphoric acid. The possibility of the nucleating of the calcium sulfate is lowered. Thus, large crystals are easily to form and a compact calcium sulfate is prevented, and the scheelite is leached effectively.

Therefore, a strategic is formed that adopting a phosphoric acid having a high concentration to decompose a scheelite, and adding a certain amount of sulfuric acid to form phosphoric acid again; by which not only problems like wrapping of tungstic acid can be solved, and tungsten is into the solution in the form of phosphotungstic acid, but also a compact calcium sulfate film is prevented, and the objective of scheelite decomposition is achieved with high efficiency. Furthermore, studies have shown that, the solubility of phosphotungstic acid is largely affected by concentrations of phosphoric acid and sulfuric acid, and the reaction temperature. This characteristic has shed light on the extraction of phosphotungstic acid from the leaching agent (for example, in a system having 20 wt. % P₂O₅ of a concentration of phosphoric acid, the relationship between the solubility of the phosphotungstic acid, the concentration of sulfuric acid, and the temperature is shown in FIG. 1, crystals formed is shown in FIG. 2). That is, in condition of an acid having a high concentration, phosphotungstic acid can be crystallized from a lixivium by cooling crystallization or concentrated crystallization, so that the tungsten is separated from the lixivium. This method can largely simplify the art flow. The mother liquor is then added with the leaching agent to an original level to carry out a next stage leaching.

In view of the above-described problems, it is one objective of the invention to provide a method for extracting tungsten from scheelite that is free of pollution, and has low cost, low energy consumption, convenient operation, and high efficiency.

To achieve the above objective, in accordance with one embodiment of the invention, there is provided a method for extracting tungsten from scheelite, the method comprising:

-   -   1) adding a mixed acid comprising sulfuric acid and phosphoric         acid into a decomposition reactor, the mixed acid comprising         150-500 g/L of H₂SO₄ and 15-35 wt. % of P₂O₅;     -   2) heating the decomposition reactor to a temperature of 70-100°         C.; adding a scheelite into the decomposition reactor and         controlling a liquid-solid ratio at 3:1-8:1 L/kg; allowing for         reaction for 1-6 h, and filtering to obtain a filtrate;     -   3) supplementing the filtrate with sulfuric acid consumed in the         reaction;     -   4) crystallizing the filtrate to obtain phosphotungstic acid         crystals and a mother liquor;     -   5) dissolving the phosphotungstic acid crystals in water to         obtain a phosphotungstic acid solution;     -   6) transforming the phosphotungstic acid solution to an ammonium         tungstate solution for preparing ammonium paratungstate (APT);         and     -   7) supplementing the mother liquor with phosphoric acid and         water to an initial level and returning the mother liquor for         ore leaching.

In a class of this embodiment, the scheelite comprises 10-75 wt. % of WO₃ and has a grain size ≦150 μm.

In a class of this embodiment, the phosphotungstic acid crystals are obtained using a cooling crystallization process by cooling the filtrate to 30-50° C. and filtering; or the phosphotungstic acid crystals are obtained using a concentrated crystallization process by concentrating the filtrate to have a ⅓-⅘ original volume, and filtering.

In a class of this embodiment, the phosphotungstic acid solution obtained from dissolving the phosphotungstic acid crystals comprises 350-500 g/L of WO₃; and the phosphotungstic acid solution is transformed into the ammonium tungstate solution comprising 200-300 g/L of WO₃ by ammonia, ion exchange, or solvent extraction.

Advantages of the invention are summarized as follows:

-   -   1. the method has no strict requirement on the content of         phosphorus in the scheelite, so that the process of phosphorus         removal is not needed in the mineral beneficiation, thereby         saving agents in phosphorus removal and lowering the loss of the         tungsten;     -   2. phosphoric acid with a high concentration can effectively         lower the degree of supersaturation of calcium sulfate during         the decomposition of tungsten ore. This is because phosphoric         acid can complex calcium ions, so that the solubility of the         calcium sulfate increases as the concentration of the phosphoric         acid increases, and the possibility of the nucleating of the         calcium sulfate is lowered. Thus, large crystals are easily to         form, and at the same time a compact calcium sulfate is         prevented, thereby effectively leaching the scheelite. The         method saves resource and energy, and has a decomposition rate         exceeding 99%;     -   3. in a leaching system comprising high concentrations of         phosphoric acid and sulfuric acid, a solubility of an obtained         phosphotungstic acid significantly varies in accordance with the         change of concentration of sulfuric acid and the temperature,         and thus the cooling crystallization process or concentrated         crystallization process can be carried out to realize the         extraction of the tungsten;     -   4. the method has overcome serious corrosion of CF and HC1         volatilization in the conventional acid decomposition;     -   5. the method realizes the recycling of phosphoric acid and the         sulfuric acid, during the process, P₂O₅ loss can be lowered to         5% below; and a consumption of sulfuric acid is equal to a         theoretical consumption of calcium in the ore, which largely         decreases the leaching cost and the discharge of the waste         water; and     -   6. the method has a short procedure, convenient operation, and         easy industrialization.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described hereinbelow with reference to the accompanying drawings, in which:

FIG. 1 is a relation chart between a solubility of phosphotungstic acid and a concentration of sulfuric acid and temperature;

FIG. 2 is an X-Ray diffraction (XRD) map of phosphotungstic acid crystals;

FIG. 3 is a flow chart of a method for extracting tungsten from scheelite in accordance with one embodiment of the invention;

FIG. 4 is an XRD map of a residue after decomposition in accordance with Example 1;

FIG. 5 is a scanning electron microscope (SEM) picture of a residue after decomposition in accordance with Example 1;

FIG. 6 is an XRD map of a residue after decomposition in accordance with comparative example 1; and

FIG. 7 is a SEM picture of a residue after decomposition in accordance with comparative example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

To further illustrate the invention, experiments detailing a method for extracting tungsten from scheelite are described below. It should be noted that the following examples are intended to describe and not to limit the invention.

EXAMPLE 1

Phosphoric acid with a high concentration is adopted which can effectively lower the degree of supersaturation of calcium sulfate during the decomposition of tungsten ore. This is because phosphoric acid can complex calcium ions, so that the solubility of the calcium sulfate increases as the concentration of phosphoric acid increases, and the possibility of the nucleating of the calcium sulfate is lowered. Thus, large crystals are easy to form and a compact calcium sulfate is prevented, and the scheelite is leached effectively. Experiments are carried out as follows:

A phosphoric acid solution having 20 wt. % of P₂O₅ was prepared, and sulfuric acid was added and mixed with the phosphoric acid solution to obtain a mixed acid solution; a concentration of H₂SO₄ was controlled at 150 g/L. Thereafter, 5 L of the mixed acid solution was added into the decomposition reactor, and was heated to a temperature of 90° C. And then 1 kg of a sheelite having 70.6 wt. % of WO₃ was added into the decomposition reactor (liquid-solid ratio was at 5:1 L/kg), and the reaction was carried out for 6 h. A leaching rate of tungsten was 99.3%. An XRD map and a SEM picture of the residue are respectively shown in FIGS. 4 and 5.

COMPARATIVE EXAMPLE 1

When a phosphoric acid having a low concentration was adopted, experiments are carried out as follow:

A phosphoric acid solution having 5 wt. % of P₂O₅ was prepared. Sulfuric acid was added and mixed with the phosphoric acid solution to obtain a mixed acid solution; a concentration of H₂50₄ was controlled at 150 g/L. Thereafter, 5 L of the mixed acid solution was added into the decomposition reactor, and was heated to a temperature of 90° C. And then 1 kg of a sheelite having 70.6 wt. % of WO₃ was added into the decomposition reactor (liquid-solid ratio was at 5:1 L/kg), and the reaction was carried out for 6 h. A leaching rate of tungsten was 87.6%. An XRD map and a SEM picture of the residue are respectively shown in FIGS. 6 and 7.

EXAMPLE 2

A phosphoric acid solution having 20 wt. % of P₂O₅ was prepared, and sulfuric acid was added and mixed with the phosphoric acid solution to obtain a mixed acid solution, a concentration of H₂SO₄ was controlled at 150 g/L. Thereafter, 6 L of the mixed acid solution was added into the decomposition reactor, and was heated to a temperature of 80° C. And then 1 kg of a sheelite having 70.6 wt. % of WO₃ was added into the decomposition reactor (liquid-solid ratio was at 6:1 L/kg), and the reaction was carried out for 6 h. A leaching rate of tungsten was 99.2%. The reaction was followed with a filter process from which a filtrate was obtained. The filtrate was condensed to a volume that was equal to ⅓ of an original volume by condensed crystallization after being supplemented for a consumed sulfuric acid. In such a condition, a crystallization rate of the phosphotungstic acid was 85.3%. After a second filter, a mother liquor was obtained; after being supplemented for phosphoric acid and the water to an original level, the mother liquor was returned for ore leaching. The phosphotungstic acid crystals were dissolved in the water to obtain a phosphotungstic acid solution having 489.3 g/L of WO₃ which was then added into an ammonia solution to obtain an ammonium tungstate solution having 250.6 g/L of WO₃.

EXAMPLE 3

A phosphoric acid solution having 15 wt. % of P₂O₅ was prepared, and sulfuric acid was added and mixed with the phosphoric acid solution to obtain a mixed acid solution, a concentration of H₂SO₄ was controlled at 300 g/L. Thereafter, 4 L of the mixed acid solution was added into the decomposition reactor, and was heated to a temperature of 90° C. And then 1 kg of a sheelite having 70.6 wt. % of WO₃ was added into the decomposition reactor (liquid-solid ratio was at 4:1 L/kg), and the reaction was carried out for 4 h. A leaching rate of tungsten was 99.5%. The reaction was followed with a filter process from which a filtrate was obtained. The filtrate was cooled to a temperature of 30° C. after being supplemented for a consumed sulfuric acid. In such a condition, a crystallization rate of the phosphotungstic acid was 82.1%. After a second filter, a mother liquor was obtained; after being supplemented for phosphoric acid and the water to an original level, the mother liquor was returned for ore leaching. The phosphotungstic acid crystals were dissolved in the water to obtain a phosphotungstic acid solution having 396.7 g/L of WO₃ which was then added into an ammonia solution to obtain an ammonium tungstate solution having 262.3 g/L of WO₃.

EXAMPLE 4

A phosphoric acid solution having 35 wt. % of P₂O₅ was prepared, and sulfuric acid was added and mixed with the phosphoric acid solution to obtain a mixed acid solution, a concentration of H₂SO₄ was controlled at 200 g/L. Thereafter, 3 L of the mixed acid solution was added into the decomposition reactor, and was heated to a temperature of 70° C. And then 1 kg of a sheelite having 70.6 wt. % of WO₃ was added into the decomposition reactor (liquid-solid ratio was at 3:1 L/kg), and the reaction was carried out for 5 h. A leaching rate of tungsten was 99.0%. The reaction was followed with a filter process from which a filtrate was obtained. After being supplemented for a consumed sulfuric acid, the filtrate was condensed to a volume that was equal to ⅘ of an original volume by condensed crystallization. In such a condition, a crystallization rate of the phosphotungstic acid was 81.7%. After a second filter, a mother liquor was obtained; after being supplemented for phosphoric acid and the water to an original level, the mother liquor was returned for ore leaching. The phosphotungstic acid crystals were dissolved in the water to obtain a phosphotungstic acid solution having 358.4 g/L of WO₃. A D301 resin was used to adsorb tungsten from the phosphotungstic acid solution, an adsorption rate of tungsten was 99.1%. After desorption by an ammonia solution, an ammonium tungstate solution having 209.3 g/L of WO₃ was obtained; and a solution after an ion exchange was used to dissolve phosphotungstic acid crystals.

EXAMPLE 5

A phosphoric acid solution having 35 wt. % of P₂O₅ was prepared, and sulfuric acid was added and mixed with the phosphoric acid solution to obtain a mixed acid solution, a concentration of H₂SO₄ was controlled at 500 g/L. Thereafter, 8 L of the mixed acid solution was added into the decomposition reactor, and was heated to a temperature of 90° C. And then 1 kg of a sheelite having 65.7 wt. % of WO₃ was added into the decomposition reactor (liquid-solid ratio was at 8:1 L/kg), and the reaction was carried out for 1 h. A leaching rate of tungsten was 99.0%. The reaction was followed with a filter process from which a filtrate was obtained. The filtrate was cooled to a temperature of 50° C. after being supplemented for a consumed sulfuric acid. In such a condition, a crystallization rate of the phosphotungstic acid was 62.7%. After a second filter, a mother liquor was obtained; after being supplemented for phosphoric acid and the water to an original level, the mother liquor was returned for ore leaching. The phosphotungstic acid crystals were dissolved in the water to obtain a phosphotungstic acid solution having 404.1 g/L of WO₃ which was then added into an ammonia solution to obtain an ammonium tungstate solution having 228.7 g/L of WO₃.

EXAMPLE 6

A phosphoric acid solution having 25 wt. % of P₂O₅ was prepared, and sulfuric acid was added and mixed with the phosphoric acid solution to obtain a mixed acid solution, a concentration of H₂SO₄ was controlled at 250 g/L. Thereafter, 4 L of the mixed acid solution was added into the decomposition reactor, and was heated to a temperature of 100° C. And then 1 kg of a sheelite having 65.7 wt. % of WO₃ was added into the decomposition reactor (liquid-solid ratio was at 4:1 L/kg), and the reaction was carried out for 3 h. A leaching rate of tungsten was 99.3%. The reaction was followed with a filter process from which a filtrate was obtained. The filtrate was cooled to a temperature of 40° C. after being supplemented for a consumed sulfuric acid. In such a condition, a crystallization rate of the phosphotungstic acid was 67.1%. The phosphotungstic acid crystals were dissolved in the water to obtain a phosphotungstic acid solution having 425.8 g/L of WO₃ which was then added into an ammonia solution to obtain an ammonium tungstate solution having 231.4 g/L of WO₃.

EXAMPLE 7

A mother liquor was obtained after the crystallization of Example 6. After being supplemented for sulfuric acid, a phosphoric acid solution, and water to make the solution having 25 wt. % of P₂O₅, and 250 g/L of a concentration of H₂SO₄, the mother liquor was used for decomposing 1 kg of a scheelite having 65.7 wt. % of WO₃. A liquid-solid ratio was controlled at 4:1 L/kg, and the reaction was carried out at a temperature of 100° C. for 3 h. A leaching rate of tungsten was 99.2%. The reaction was followed with a filter process from which a filtrate was obtained. The filtrate was condensed to a volume that was equal to ½ of an original volume by condensed crystallization after being supplemented for a consumed sulfuric acid. In such a condition, a crystallization rate of the phosphotungstic acid was 86.4%. After another filter, a new mother liquor was obtained; after being supplemented for phosphoric acid and water to an original level, the new mother liquor was returned for ore leaching. The phosphotungstic acid crystals were dissolved in water to obtain a phosphotungstic acid solution having 367.3 g/L of WO₃ which was then added into an ammonia solution to obtain an ammonium tungstate solution having 253.8 g/L of WO₃, 2.4 g/L of P, and 25.6 g/L of SO₄ ²⁻. An ammonium magnesium salt method was adopted to remove impurities. A MgCl₂ solution having 200 g/L of MgCl₂ was added according to 1.2 of a mol ratio of Mg to P, the reaction was maintained for 30 min at a room temperature, then a filter was carried out, after which a removal rate of phosphorus was 99.9%, and a loss of tungsten was only 0.05 wt. %. Mo was removed according to a method disclosed in Pat. No. 97108113.1, and a consequent solution was crystallized by volatilization to obtain an ammonium paratungstate (APT) crystals. The crystallization rate of APT was 94.5%. Analysis results of the products are shown in Table 1.

TABLE 1 Analysis results of an APT having a crystallization rate of 94.5 wt. % Content Content Element (wt. %) Element (wt. %) Element Content (wt. %) P 0.00043 Mg <0.0007 Bi <0.0001 K <0.001 Ni <0.0005 Sn <0.0001 Na 0.001 Ti <0.0005 Sb 0.0002 Mo 0.0034 V <0.0005 Cu <0.0001 Al <0.0005 Co <0.0005 Ca <0.0005 Si <0.0005 Cd — Cr <0.0010 Mn <0.0005 Pb <0.0001 As 0.0007

EXAMPLE 8

A phosphoric acid solution having 35 wt. % of P₂O₅ was prepared, and sulfuric acid was added and mixed with the phosphoric acid solution to obtain a mixed acid solution, a concentration of H₂SO₄ was controlled at 300 g/L. Thereafter, 5 L of the mixed acid solution was added into the decomposition reactor, and was heated to a temperature of 90° C. And then 1 kg of a sheelite having 45.9 wt. % of WO₃ was added into the decomposition reactor (liquid-solid ratio was at 6:1 L/kg), and the reaction was carried out for 6 h. A leaching rate of tungsten was 99.0%. The reaction was followed with a filter process from which a filtrate was obtained. The filtrate was condensed to a volume that was equal to ⅔ of an original volume by condensed crystallization after being supplemented for a consumed sulfuric acid. In such a condition, a crystallization rate of the phosphotungstic acid was 84.7%. After a second filter, a mother liquor was obtained; after being supplemented for phosphoric acid and the water to an original level, the mother liquor was returned for ore leaching. The phosphotungstic acid crystals were dissolved in the water to obtain a phosphotungstic acid solution having 376.8 g/L of WO₃ which was then extracted using a primary amine as an extractant, and an extraction rate of tungsten was 98.8%. Thereafter, an ammonia solution was used to stripping to obtain an ammonium tungstate solution having 205.2 g/L of WO₃. A liquor from the extraction was used to dissolve the phosphotungstic acid crystals.

EXAMPLE 9

A phosphoric acid solution having 25 wt. % of P₂O₅ was prepared, and sulfuric acid was added and mixed with the phosphoric acid solution to obtain a mixed acid solution, a concentration of H₂SO₄ was controlled at 500 g/L. Thereafter, 3 L of the mixed acid solution was added into the decomposition reactor, and was heated to a temperature of 90° C. And then 1 kg of a sheelite having 30.4 wt. % of WO₃ was added into the decomposition reactor (liquid-solid ratio was at 3:1 L/kg), and the reaction was carried out for 4 h. A leaching rate of tungsten was 98.9%. The reaction was followed with a filter process from which a filtrate was obtained. The filtrate was condensed to a volume that was equal to ⅓ of an original volume by condensed crystallization after being supplemented for a consumed sulfuric acid. In such a condition, a crystallization rate of the phosphotungstic acid was 72.9%. After a second filter, a mother liquor was obtained; after being supplemented for phosphoric acid and the water to an original level, the mother liquor was returned for ore leaching. The phosphotungstic acid crystals were dissolved in the water to obtain a phosphotungstic acid solution having 364.7 g/L of WO₃ which was then added into an ammonia solution to obtain an ammonium tungstate solution having 225.1 g/L of WO₃.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. 

The invention claimed is:
 1. A method for extracting tungsten from scheelite, the method comprising the steps of: 1) adding a mixed acid comprising sulfuric acid and phosphoric acid into a decomposition reactor, the mixed acid comprising 150-500 g/L of H₂SO₄ and 15-35 wt. % of P₂O₅; 2) heating the mixed acid to a temperature of 70-100° C.; adding scheelite to the decomposition reactor and controlling a liquid-solid ratio at 3: 1-8:1 L/kg; allowing components in the decomposition reactor for reaction for 1-6 h, and filtering to obtain a filtrate; 3) supplementing the filtrate with sulfuric acid consumed in the reaction; 4) crystallizing the filtrate to obtain phosphotungstic acid crystals and mother liquor; 5) dissolving the phosphotungstic acid crystals in water to obtain a phosphotungstic acid solution; 6) transforming the phosphotungstic acid solution to an ammonium tungstate solution for preparing ammonium paratungstate (APT); and 7) supplementing the mother liquor with phosphoric acid and water to an initial level and returning the mother liquor for ore leaching.
 2. The method of claim 1, wherein the scheelite comprises 10-75 wt. % of WO₃ and has a grain size ≦150 μm.
 3. The method of claim 1, wherein the phosphotungstic acid crystals are obtained using a cooling crystallization process by cooling the filtrate to 30-50° C. and filtering; or the phosphotungstic acid crystals are obtained using a concentrated crystallization process by concentrating the filtrate to have a ⅓-⅘ original volume, and filtering.
 4. The method of claim 1, wherein the phosphotungstic acid solution obtained from dissolving the phosphotungstic acid crystals comprises 350-500 g/L of WO₃; and the phosphotungstic acid solution is transformed into the ammonium tungstate solution comprising 200-300 g/L of WO₃ by adding ammonia, ion exchange, or solvent extraction.
 5. The method of claim 3, wherein the phosphotungstic acid solution obtained from dissolving the phosphotungstic acid crystals comprises 350-500 g/L of WO₃; and the phosphotungstic acid solution is transformed into the ammonium tungstate solution comprising 200-300 g/L of WO₃ by adding ammonia, ion exchange, or solvent extraction. 